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@gasket/engine

Plugin engine for Gasket. This is used internally by the Gasket CLI.

What is a Plugin?

A plugin a a module in the gasket ecosystem that provides a unit of functionality. Some plugins are core to the overall application, others optional.

For documentation on plugins, refer to the plugin guide.

What is a Preset?

A preset is simply a package with Gasket plugins dependencies. This allows common plugins to be grouped together, and loaded by way of a preset.

For documentation on presets, refer to the preset guide.

GasketAPI

The GasketAPI object passed to hook functions has the following members:

config

The Gasket configuration object, typically derived from gasket.config.js.

exec(event, ...args)

The exec method enables a plugin to introduce new lifecycle events. When calling exec, await the Promise it returns to wait for the hooks of other plugins to finish. If the lifecycle event is for producing data, the Promise will contain an Array of the hook data stored in the order the hooks were executed.

execSync(event, ...args)

The execSync method is like exec, only all hooks must execute synchronously. The synchronous result is an Array of the hook return values. Using synchronous methods limits flexibility, so it's encouraged to use async methods whenever possible.

execMap(event, ...args)

The execMap method is just like exec, only the Promise result is an object map with each key being the name of the plugin and each value the result from the hook. Only the plugins that hooked the event will have keys present in the map.

execMapSync(event, ...args)

The execSyncSync method is like execMap, only all hooks must execute synchronously.

execApply(event, async applyFn)

The execApply method execution is ordered like exec, but you must invoke the handler yourself with explicit arguments. These arguments can be dynamic based on the plugin itself. e.g.:

await gasket.execApply('someEvent', async (plugin, handler) => {
  // Creating the "Contextual thing" can be sync or async. async params are
  // fully supported with this pattern if necessary.
  const arg = await Contextual.thingWith(plugin);
  return handler(arg); // The gasket parameter is automatically applied
});

execApplySync(event, applyFn)

The execApply method is like execApply, only all hooks must execute synchronously.

execWaterfall(event, value, ...args)

The execWaterfall method is like exec, only it allows you to have each hook execute sequentially, with each result being passed as the first argument to the next hook. It's like an asynchronous version of Array.prototype.reduce. The final result is returned in the resulting Promise.

execWaterfallSync(event, value, ...args)

The execWaterfallSync method is like execWaterfall, only each hook must execute synchronously. The final value is returned synchronously from this call. Using synchronous methods limits flexibility, so it's encouraged to use async methods whenever possible.

hook({ event, handler, timing, pluginName })

Injects additional lifecycle hooks at runtime. Takes a single object parameter with the following properties:

Property Required? Description
event Yes The name of the event to hook. This is the same thing as the property name in the hooks of a plugin definition.
handler Yes The function to call when the event occurs. The function should take the same form as the hooks callbacks in a plugin definition.
timing No Ordering constraints for when the hook will execute. Same as the optional timing property in plugin hooks.
pluginName No Defaults to an auto-generated name. Only supply this if you need other hooks to be able to order themselves relative to this hook via timing constraints. Important note: only one hook per event is allowed per plugin name, so if your plugin is injecting dynamic hooks, be sure that the names are dynamic enough to avoid conflicts.

Plugin priority

When a preset extends another preset, the version of the plugin registered can be overridden if the parent preset sets a different dependency version. Additionally, if an app specifies a plugin directly in the gasket.config using add, the version determined by the app will instead be registered.

Direct Usage

This package is used internally by the Gasket CLI, so you probably do not need to use this directly. If you do, instantiate an engine, passing in the gasket.config.js export.

const PluginEngine = require('@gasket/engine');
const gasketConfig = require('../gasket.config');

const pluginEngine = new PluginEngine(gasketConfig, context);

The plugin engine will parse the configuration, import the configured plugins, and register their hooks. The engine instance is the same GasketAPI object passed to hook handlers.

PluginEngine context

Execution of plugins through a PluginEngine instance is dependent on the context of that operation. Currently the context consists of:

  • resolveFrom: absolute path to prepend to any moduleName before attempting resolution with require.

e.g.

const PluginEngine = require('@gasket/engine');

const path = require('path');
const resolveFrom = path.resolve('./someapp');
const gasketConfig = require(path.join(resolveFrom, 'gasket.config'));

const pluginEngine = new PluginEngine(gasketConfig, { resolveFrom });

The above will resolve all Plugins and Presets from within ./someapp instead of resolving relative to the current directory.

Tracing

The Gasket engine uses the debug package to trace the plugin lifecycle. If you set the DEBUG environment variable to gasket:engine you'll see additional output in stderr indicating when things are invoked.

License

MIT